This invention relates generally to variable-scale photocopying machines of the type in which the scale of the size of a photocopy relative to that of its original can be varied thereby to obtain an enlarged or reduced copy of the original. More particularly, the invention relates to an aperture-controlling device for controlling, in accordance with the scale of the photocopying, the quantity of the photocopying exposure light projected onto the photosensitive medium in a variable-scale photocopying machine.
In a variable-scale photocopying machine of the character referred to above, the rotational speed of the drum bearing the photosensitive medium is the same for enlargement and for reduction. At the time of reduction, however, whereas the quantity of light per unit area of exposure increases because the size of the projected image is reduced by changing the lens or otherwise changing the lens focus, the quantity of light decreases since the scanning speed increases. Furthermore, because of deviations in the transmittance of the lens system, the reflectivity of the mirror, and the lens angle of view from machine to machine of the same type, the quantity of light projected onto the photosensitive drum tends to deviate from the optimum light quantity. These problems are encountered also at the time of enlargement.